Cross-rifled vapor generating tube

ABSTRACT

A vapor generating tube of the two-phase type which is operated at a pressure between sub-critical and critical pressure of the vapor has a tube-like body through which fluid is passed. The inner wall surface of the tube is crossed-rifled to provide a plurality of quadrilaterally shaped projections uniform and regularly spaced along the cross-rifled axes. The minor inside diameter d1, the projection height h of each projection, the projection pitch P, projection width b, and spiral lead angle Alpha and Beta have the following relationships: P/h is from 5 to 4 h/d1 is from 0.005 to 0.08, b/P is in the range of 0.2 to 0.8, and the spiral lead angles Alpha + Beta are from 20* to 80* .

United States Patent 1 Nakamura et al.

[ 1 May 22, 1973 [54] CROSS-RIFLED VAPOR GENERATING TUBE [73] Assignee:Sumitomo Metal Industries Limited,

Osaka, Japan [22] Filed: July 1, 1970 [21] Appl. No.: 51,434

[30] Foreign Application Priority Data July 2, 1969 Japan ..44/52752[52] US. Cl ..138/177 [51] Int. Cl ..F16l 9/00 [58] Field of Search.....138/32-38, 177; 165/109 T, 165/179 [56] References Cited UNITED STATESPATENTS 2,537,797 l/l951 Simpelaar ..l65/l79 3,088,494 5/1963 Koch etal. ..l38/37 2,663,321 12/1953 Jantsch .4 2,080,626 5/1937 Mojonnier138/38 Primary Examiner-Houston S. Bell, Jr. Att0rneyWatson, Cole,Grindle & Watson [57] ABSTRACT A vapor generating tube of the two-phasetype which is operated at a pressure between sub-critical and criticalpressure of the vapor has a tube-like body through which fluid ispassed. The inner wall surface of the tube is crossed-rifled to providea plurality of quadrilaterally shaped projections uniform and regularlyspaced along the cross-rifled axes. The minor inside diameter (1,, theprojection height h of each projection, the projection pitch P,projection width b, and spiral lead angle a and B have the followingrelationships: P/h is from 5 to 4 h/d is from 0.005 to 0.08, b/P is inthe range of 0.2 to 0.8, and the spiral lead angles a+B are from 20 to80 4 Claims, 4 Drawing Figures TEMPERATURE PATENTED 3,784,140

SHEET 2 BF 3 FIG.3

o smoom TUBE L we FLUX 4on0 kccfl/m -s Q2058 RLFLEU TUBE HEAT FLUXSOXK)4 kcai/m -hr 500 crzoss ram-:0 TUBE HEAT Fl ulr 1h 4on0 m r SMOOTHTUBE M CG HEAT FLUX X\0 kcd lm hr SMOOTH TUBE 400- t HEAT FLUX \0x\0kcollm' -hr CROSS mFLED TUBE HEAT FLUX 3on0 kco! /m"-hr CROSS Yaw-LEDTUBE HER FLUX QOX\O kc0\/m .hr

QUMJTY ENTHALPY i kcal/ kg INVENTORSI ///5,45H/ M/(flMU/(A Mas/47mm 74mmCROSS-RIFLED VAPOR GENERATING TUBE This invention relates toimprovements in vapor generating tubes to be operated mostly under apressure below the critical pressure and to be subjected to high heatflux, and more particularly to a cross-rifled vapor generating tubeprovided uniformly with many projections of a rhombic or parallelogramicshape made on the inside wall surface by cross-rifling the insidesurface.

More particularly, the above mentioned uniform projections are regularlyset by cross-rifling the inside surface of a tube to be used mostly as awall tube for a high temperature-high pressure boiler using a fossilfuel (such as coal, heavy oil or natural gas), so that there is produceda remarkable improvement in the critical heat flux so as to be higherthan in a smooth tube by promoting the maintenance of nucleate boilingof the fluid passing through the tube.

Further, not only in two-phase flow but also in singlephase flow, theheat transfer is remarkably promoted. Therefore, when the cross-rifledtubes of this invention are used for general heat-exchangers having noboiling or vapor generating tubes operated under super-criticalpressure, a remarkable effect improvement is obtained.

The safety of the operation of vapor generating tubes of a boilerdepends on the wall temperature of the tube. That is to say, it isrelated so closely with the coefficient of heat transfer on the insidesurface of the tube, the temperature of the operating liquid (such asthe mixture of steam and water) and the heat flux that it is necessaryto always pay careful attention to the wall surface exposed to strongflame radiation during its operation. Today, due to the increase of theadoption of an oil burning system and the general use of a oncethroughboiler, there has increased the possibility of the local presence of acondition exceeding in some cases the critical heat flux causingphysical burn-out (breakdown) of the vapor generating tube. As acountermeasure for increasing the critical heat flux, a tube called aribbed tube made by forming spiral lands on the inside surface of avapor generating tube is suggested, for example, in U.S. Pat. No.3,088,494 (or Canadian Pat. No. 684,836).

An object of the present invention is to provide a cross-rifled vaporgenerating tube having the effect of remarkably improving the criticalheat flux to be higher than in a smooth tube by promoting themaintenance of nucleate boiling of the fluid passing through the tube.

Generally, the burn-out phenomenon of a vapor generating tube subjectedto high heat flux and used below the critical pressure is thought toconsist of two factors of fast burn-out (also called heat flux burn-out)in a low quality region and slow burn-out (also called enthalpyburn-out) in a high quality region. The former is caused by thetransition from nucleate boiling to fllm boiling but the latter iscaused by the annular flow pattern of the gas-liquid mixture. Thus twokinds of burn-out of the vapor generating tube are caused by quitedifferent mechanisms. Therefore, in order to make the critical heat fluxin the vapor generating tube definite, it is an essential condition tomeasure the critical heat flux by heat transmission tests and to watchthe flow pattern by flow tests.

1n the drawings:

FIG. 1 is a sectional view of an embodiment of a cross-rifled vaporgenerating tube according to the present invention;

FIG. 2 is a sectional view showing another embodiment of the tube shownin FIG. 1;

FIG. 3 is a graph showing a comparison of inside sur face temperaturesof a smooth tube and a cross-rifled tube of the present invention in thecase of a pressure of 210 atmo (Kg/cm and a mass velocity of 700 to 710l(g./m see;

FIG. 4 is a graph showing a comparison of the maximum values of theinside surface temperatures to the heat fluxes of a smooth tube and across-rifled tube of the present invention in the case of a pressure of210 atmo (Kg/cm);

The cross-rifled tube according to the present invention has manyprojections such as 18 or 20 formed on the inside surface of a tube asshown in the embodiment in either FIGS. 1 or 2.

Examples of the chemical compositions mechanical properties anddimensions of the test tubes are shown in the following Tables 1 and 2.

TABLE 1 Chemical compositions and mechanical properties of cross-rifledtubes of the present invention.

Chemical compositions (in percent by weight.)

Test tubes 0 Si M11 1 S Ni C1 Mo A 0. 0S 0. 57 1. T4 0. 024 0. 006 13.40 10. 50 1.. 20 B 0. 07 0. 54. 1. 08 0. 026 0. 006 E). 60 18. 50 0. 07

Mechanical properties Test Tubes Tensile strength Yield point (in Kglmm(in Kg/mm) A 56.3 28.8 B 51.4 27.2

Mechanical properties Test Tubes Elongation (in TABLE 2:

Dimensional data of cross-rifled test tubes Test tubes A-l A2 B-l 8-2Outside diameter D, (in mm) 20.27 20.16 20.09 20.06 Minor insidediameter d, (in mm) 9.66 9.54 13.27 13.18 Height h (in mm) of eprojection 0.55 0.64 0.47 0.52 Number of spiral 12 12 12 12 (number percross-section) Width 11 (in mm) of the projection 3.16 3.16 3.75 3.75Lead angle 0: 2220 2220 2148 2148 Lead angle [3 1845 1845 1837 1837 Leadangle a 5 4105 4105 4025 4025 littlll p (in llllll.)[=l(:t(l (in lltight7.32 7.32 11. .20 11.20 1n m.)Xnlu n|u-r of spisnls] [Left 1;.88 ti. 888.115 8. ()5 11:11.10 (.OIHHUOIIZ WI lltl'tll 13.30 11.42 23. 21.60 i(L011. 12. 50 10. 7X 18. 40 1G. (15 ll/ll1 4 0. 057 0. 007 0. 036 0. 010 b/IL iltlgllt U. 432 U. 432 1). 335 0. 335 ll. 0. 45'.) 0. 151) 0.433 0. 433

The inside surface temperatures of the smooth tube and the cross-rifledtube of the present invention in the case of a pressure of 210 atmo(Kg/cm) and a mass velocity of 700 to 710 kg/m sec. are shown for bulkaverage specific enthalpy with the heat flux as a parameter in FIG. 3.In the same graph, the heat flux q is taken in the range of X 10' to 50X 10 kcal/m hr., the test data of the inside surface of the smooth tubeare shown with solid lines and those of the cross-rifled tube accordingto the present invention are shown with dotted lines to compare therelative effects and a quality scale is also given on the abscissa forreference. As is obvious from this graph, in both a single-phase regiona twophase region, in the case of the cross-rifled tube, the insidesurface temperature is much lower than in the case of the smooth tubeand the heat transfer is promoted. That is to say, even in the region inwhich the wall temperature rises quickly in the case of the smooth tube,the nucleate boiling is kept up to the high quality region in the caseof the cross-rifled tube. Therefore, the wall temperature of thecross-rifled tube is much lower than of the smooth tube.

Further, as can be seen from FIG. 3, the quick rise of the walltemperature in the case of the smooth tube begins below a quality of50%. It is judged from these test results that the burn-out occurssubstantially in the state of bubble flow and is so-called fast burn-outby the transition from nucleate boiling to film boiling.

The maximum value (Tw max.) of the wall temperature vs. the heat flux atthat time as shown with the mass velocity as a parameter is as in FIG.4. In this graph, the smooth tube and the cross-rifled tube of thepresent invention are compared with each other for the three conditionsof mass velocities of 900, 700 and 400 kg/m sec. In the case of thecross-rifled tube of the present invention, the above mentioned maximumvalue (Tw max.) of the wall temperature is much lower than in the caseof the smooth tube and, in this respect, too, the effect of thecross-rifled tube is shown to be remarkable.

It is shown that the cross-rifled tube the present invention can wellendure physical burn-out at a heat flux of 60 X 10 kcallm hr. even undersuch severe condition as ofa mass velocity of 400 kg/m. sec. As themaximum local heat flux of an oil burning oiler is 50 to 60 X 10 kcal/m.hr., if such cross-rifled tube is used in high heat flux parts of anordinary boiler using a fossil fuel, there is no danger of a physicalburn-out and it contributes much to the design of a subcritical pressureboiler.

Further, the superiority of this cross-rifled tube can be proved evenfrom the results of the two-phase airwater flow test at normaltemperature and pressure. That is to say, according to the flow test, inthe crossrifled tube, in the case of bubble flow, bubbles are morelikely to concentrate in the center part of the tube than in the smoothtube and the ribbed tube of the prior art and, in the case of annularflow, the water film thickness becomes larger. Therefore, it is thoughtthat the cross-rifled tube of this invention is superior to tubes of anyother type and shape in both fast burn-out and slow burn-out and enablesan increase in the critical heat flux. Further, what is to bespecifically noted is that the pressure drop in the cross-rifled tube ina single-phase flow and two-phase flow is small. This a remarkablesuperiority to the ribbed tube having a large lead angle.

The cross-rifled tube of the present invention is made by using acold-drawing process with a die and plug. First of all, as a first stepworking, a plug on which a plurality of spiral grooves are made inadvance is inserted into a tube and a plurality of spiral lands areformed on the inside surface of a smooth tube which is a mother tube bythe free rotation of this plug and then as a second step working,another plug on which a plurality of spiral grooves with the lead anglereversed are made or on which straight grooves are made is inserted intothe above mentioned tube in which the spiral lands are made and a partof the spiral lands made in said tube is plastically pressed down by thefree rotation or straight drawing of this plug so that many projectionsof a rhombic or any shape may be uniformly and discontinuously made onthe inside surface of the tube.

As mentioned above, many projections are set on the inside surface ofthe tube. The shape of the projection is different depending on theshape of the plug to be used or the combination of such dimensional dataas the percentage of the area reduction.

From the results of broad tests, it has been found that the shape andarrangement of the projections of the cross-rifled tube of thisinvention are very advantageous to the heat transfer effect. As aresult, it has been found that, in order that the cross-rifled tube ofthe present invention may maintain nucleate boiling from a low qualityregion to a high quality region for such given conditions as pressure,heat flux and mass velocity, it should have projections (landssatisfying the following rations and arrangement conditions:

P/h 5 to 40, h/d, 0.005 to 0.08

b/p 0.2 to 0.8 and a B 20 to 80 wherein P represents a pitch of theprojections deter mined by the number of spirals determined by the leaddistances of the lead angles a and B of the spirals and onecross-section, h represents a height of the projection, d represents aminor inside diameter of the tube and b represents a width of theprojection as projected in the radial direction of the tube. Further,the shape and arrangement of the projections selected to obtain the bestresults in maintaining nucleate boiling should satisfy the followingconditions:

P/h 8 to 25,

h/d 0.01 to 0.07

b/P= 0.3 to 0.6 and a+B=30to In the above mentioned limitations of therespective magnitudes, the numbers of spirals counted in onecross-section and convenient to provide in the manufacture are 6, 12, 18and 24 spirals. However, in the tube of the present invention, 12 and I8spirals are used. In the above mentioned Table 2, the respective ratioconditions of four kinds of test tubes are shown. As shown in the table,the respective test tubes are satisfactory within the range of all theratio and arrangement conditions. No great difference is recognized atall in the test results of these tubes. Among the above mentionedconditions, only the last mentioned arrangement condition of a B 20 tois a condition determined by the cold-drawing by the free rotation ofthe plug, that is,the limitation in the manufacture. The setting of thespiral lands on the inside surface of the tube by the free rotation ofthe plug is determined by the frictions between the plug and die and thetube and the maximum limit of each of a and B is 43.

The shape and arrangement of the projections in the cross-rifled tubebringing about the maximum critical heat flux are different depending onthe pressure to be used. However, it has been confirmed that, in theabove mentioned embodiment of a sub-critical pressure of 210 atmo(Kg/cm") in the case of a mass velocity of 700 Kg/m" sec. and a heatflux of 60 X kcal/m hr., nucleate boiling is maintained until a qualityof 70 percent. Further, at a mass velocity of 400 kg/m sec,, it ispossible to keep the allowable temperature of an ordinary boiler tubebelow 500C. under such severe condition as heat flux of 60 X 10 kcal/mhr. By using a cross-rifled tube of the present invention, almost allthe problems in the design of the boiler tube are obviated.

What is claimed is:

1. A vapor generating tube of the two-phase type operated at a pressureabove sub-critical pressure and below critical pressure of the vapor andsub-jected to high heat flux, comprising;

a tube-like body for passing fluid therethrough and having an inner wallsurface, said inner wall surface is cross-rifled and has a plurality ofprojections uniformly and regularly spaced thereon along thecross-rifling axes, and each of said projections hav ing a quadrilateralshape.

2. A vapor generating tube as in claim 1 wherein said tube-like body hasa minor inside diameter d and each of said projections have a projectionheight h, a projection pitch P, and a projection width b, wherein P/h isin the range of 5 to 40, h/d, is in the range of 0.005 to 0.08, and b/Pis in the range of 0.2 to 0.8, the spiral lead angles of thecross-riflings are a and B and a B is in the range of 20 to 80.

3. A vapor generating tube as in claim 2 wherein P/h is in the range of8 to 25, h/d is in the range of 0.01 to 0.07, b/P is in the range of 0.3to 0.6 and a B is in the range of 30 to 4. A vapor generating tube as inclaim 2 wherein 20 a 43 and B 0.

1. A vapor generating tube of the two-phase type operated at a pressureabove sub-critical pressure and below critical pressure of the vapor andsub-jected to high heat flux, comprising; a tube-like body for passingfluid therethrough and having an inner wall surface, said inner wallsurface is cross-rifled and has a plurality of projections uniformly andregularly spaced thereon along the cross-rifling axes, and each of saidprojections having a quadrilateral shape.
 2. A vapor generating tube asin claim 1 wherein said tube-like body has a minor inside diameter d1,and each of saiD projections have a projection height h, a projectionpitch P, and a projection width b, wherein P/h is in the range of 5 to40, h/d1 is in the range of 0.005 to 0.08, and b/P is in the range of0.2 to 0.8, the spiral lead angles of the cross-riflings are Alpha andBeta , and Alpha + Beta is in the range of 20* to 80*.
 3. A vaporgenerating tube as in claim 2 wherein P/h is in the range of 8 to 25,h/d1 is in the range of 0.01 to 0.07, b/P is in the range of 0.3 to 0.6and Alpha + Beta is in the range of 30* to 75*.
 4. A vapor generatingtube as in claim 2 wherein 20* Alpha <43* and Beta 0*.